The epoxidation of olefins, such as propylene to propylene oxide (“PO”), on the industrial scale is among the most challenging chemical processes. Propylene oxide is mainly used for the production of propylene glycol and polyester, and is the starting material for polyurethane, unsaturated resins, and other products.
Industrially over 4.5 million tons of propylene oxide is produced each year using either the chlorohydrin or Halcon process. In the chlorohydrin process, propylene reacts with chlorine and water to produce 1-chloro-2-propanol and HCl, which is then treated with base to generate propylene oxide and salt. There are two pounds of salt waste for each pound of propylene oxide produced. The process consumes large amounts of chlorine and lime which are finally converted to useless and environmentally polluting waste.
The Halcon or hydroperoxide process, organic peroxides are produced, and epoxidation is performed in the presence of transition metal catalysts leading to the formation of propylene oxide and co-products. The economic viability of this process depends on the market value of the co-products because 3 to 4 times more co-product is produced than the weight of propylene oxide produced. Direct oxidation of propylene to propylene oxide with oxygen would be highly desirable, but the presence of propylene's highly reactive allylic hydrogens renders this approach quite difficult.
Minimizing waste in the selective oxidation of propylene to propylene oxide has long been an important objective of industrial chemistry. So-called titanium-substituted silicalite (“TS-1”) catalysts can catalyze this reaction with reasonable efficiency using aqueous hydrogen peroxide (H2O2) and gaseous O2/H2. TS-1 catalysts have high catalytic activity and selectivity. However, the method is problematic because the catalyst is rapidly deactivated and high temperature is required to regenerate the catalyst. U.S. Pat. No. 5,155,247 entitled “Use of Organorhenium compounds for the oxidation of multiple C—C bonds, oxidation processes based thereon and novel organorhenium compounds” to Herrmann et al. employed a methyltrioxorhenium (CH3ReO3, hereafter identified as “MTO”) catalyst to alkene oxidation under low temperature (below 10° C.) conditions, using hydrogen peroxide as the oxidant. This low temperature process produced propylene oxide and propylene glycol (“PG”) in the ratio of 1:1. Herrmann focused on anhydrous hydrogen peroxide (preferably in tert-butanol as a solvent) because water was detrimental to hydrolytic epoxide ring opening. In U.S. Pat. No. 5,166,372 entitled “Epoxidation Process,” Crocco et al. improved the selectivity for propylene oxide by employing an alkyl aryl secondary alcohol in the reaction mixture. Magnesium sulfate was used to remove water from the system. In Sharpless, U.S. Pat. No. 5,939,568, nitrogenous aromatic heterocycles were employed as “accelerants” in methylene chloride solvent systems. Lastly, in Sharpless, U.S. Pat. No. 6,271,400, anhydrous oxidants (trialkyl silyl peroxides) and water removal agents were used to reduce the water concentration in the reaction mixture.
Ethylene oxide is a widely used industrial organic intermediate with a worldwide demand in 2006 of 18 million tons, and anticipated annual growth rate of 1.7%. Currently ethylene oxide technology involves either air or O2 oxidation of ethylene using a silver-based catalyst. Typically, the ethylene conversion is maintained at a low level to minimize combustion products. The maximum ethylene conversion is in the range of 10-15% with less than 10% being more typical, and the maximum ethylene oxide selectivity is 85-90%, the byproduct being mainly carbon dioxide formed from the combustion of both ethylene and ethylene oxide. It has also been reported that up to 30% of the ethylene converted may undergo combustion to form carbon dioxide and water (EPA-450/4-84-007L). Among industrial chemical technologies, the ethylene oxide process is one of the largest emitters of carbon dioxide as the ethylene oxide byproduct. Since ethylene accounts for 70-80% of the cost of the product, the burning of ethylene and the product result in value destruction, the generation of waste, and an environmental hazard. Hence, the development of robust catalysts that maximize ethylene oxide selectivity continues to be of paramount interest to industry.
Safety is a major concern in conventional ethylene oxide processes due to the potential for formation of explosive ethylene/ethylene oxide/air mixtures in the gas phase under reaction conditions. Ethylene oxide is a highly explosive substance with a lower flammability limit of 3 mol % and an upper flammability extending to pure ethylene oxide, which is susceptible to spontaneous explosive decomposition by a radical mechanism. Thus, it would be desirable to have an alternate ethylene oxide technology that is selective only towards ethylene oxide (thus eliminating the formation of CO2 as a byproduct), while for safety reasons, also avoiding the formation of explosive ethylene oxide/oxygen mixtures in the gas phase. Such a process would significantly reduce the carbon footprint of this large-scale industrial process.
The present invention is directed to an olefin oxide synthesis process, such as for the production of propylene oxide and ethylene oxide, which has a number of advantages over the prior art.